From Waste to Resources: Sewage Sludges from the Citrus Processing Industry to Improve Soil Fertility and Performance of Lettuce (Lactuca sativa L.)

Abstract

The citrus industry produces a large number of sludges as a consequence of citrus wastewater treatment. The correct disposal of citrus sewage sludges (CSSs) has been attempted using anaerobic digestion, aerobic digestion, and lime stabilization. However, since CSSs hold nitrogen, phosphorus, and other macronutrients required by crops, in line with the circular economy principles, they could be utilized for agricultural purposes, such as organic fertilizer. The aim of this study was to assess the effect of CSSs supplied at different doses on soil fertility and lettuce performance. To this end, a pot experiment was established. The soil was amended with CSSs at three different concentrations (2.5, 5, 10 t ha−1). After 46 days of lettuce growth, the experiment was stopped, and soils and plants were analyzed. Soil amended with CSSs showed an increase in total organic C ranging from 7% to 11%. Additionally, available P increased but only at the highest CSS dose. The addition of CSSs affected the biochemical properties of soil, but a univocal trend related to the number of CSSs applied was not found. Microbial biomass C increased only with the highest dose of CSS applied, while the metabolic quotient (qCO2) decreased. Such a positive effect on soil fertility and soil microorganisms, in turn, lead to an increase in lettuce biomass. Moreover, results indicated that following CSS addition, lettuce crops adsorbed more N in leaves than in roots, whereas P, Ca, Mg, K, and Na showed an opposite pattern and increased more consistently in roots. In conclusion, amendment with CSSs enhances soil fertility by increasing, regardless of CSS dose, total organic C, and, at the highest dose, P availability and microbial biomass C. Such improvement in soil fertility, in turn, increases lettuce biomass production without affecting its quality, i.e., alteration of the (K + Na)/(Ca + Mg) ratio.

Soil Quality and Crop Nutrition

“…Moreover, the reuse fits with the principle of circular economy as aimed by European Commission. In line with the study of Lucia et al [2], Wichrowska and Szczepanek [6] compared the effect of a bio-fertilizer in affecting potato quality with farmyard manure (FYM) and organic and mineral fertilizers. The tested biofertilizer consisted of yeast, lactic acid bacteria (Lactobacillus and Lactococcus), Pseudomonas, Actinobacteria, and Azotobacter as well as small amounts of micro-and macronutrients.…”

“…The article by Lucia et al [2] aimed at investigating the effect of sewage sludge deriving from the citrus processing industry on soil fertility and lettuce growth performance. In a pot experiment, lettuce was cultivated using a soil amended or not with different amounts of citrus sewage sludge.…”

Biomass Assessment and Optimization of Alcaligenes faecalis Isolated from some Nigerian Mining Sites for Heavy Metal Uptake Using Response Surface Methodology Model

“…Research indicates that heavy metals with no known biological function, such as Cd and As, are hazardous even at low concentrations (Bharti and Sharma, 2022), while heavy metals that operate as co-factors may be necessary in small quantities but become poisonous at higher doses (Lucia et al, 2023). Some heavy metals, like zinc (Zn), iron (Fe), copper (Cu), cobalt (Co), and molybdenum (Mo), are needed by humans in trace amounts but can be harmful in higher concentrations (Balali-Mood et al, 2021).…”